1. Field of Invention
The invention relates to the field of tension-stiffening devices and surgical instrumentation. More specifically, it relates to a variable-pitch cam mechanism for use in a malleable-shaft surgical retractor, particularly those that have a fiber running through their length and stiffen upon the tensioning of that fiber.
2. Description of Related Art
In the field of medical instrumentation, malleable shaft stabilization devices are known in the art. Particularly in Coronary Artery Bypass Graft (CABG) procedures, such instruments are used to stabilize the surface of the beating heart muscle at the site where the anastomosis will be formed. The instruments will have a malleable shaft of a certain length, with a stabilizer foot at a distal end. The shaft can be positioned as required, then a lead screw at a proximal end is turned by some manner of handle. The turning of the lead screw applies tension to a fiber, such as a metal wire or braided cable. The tensioning of the cable compresses the malleable shaft, and secures it rigidly into position. These devices are considered an improvement over other stabilizers, because the length of the shaft can be positioned away from the surgical field, as compared to fixed-arm stabilizers that may impede access and vision. With a malleable shaft stabilizer, the surgeon's hands are free to perform another task, and/or the surgical field is clear of obstruction.
These devices are not without drawbacks, however. Among these are the difficulties with the tensioning method. When tensioning the fiber, the forces on the fiber are relatively light as tension is first applied, but comparatively higher at the end of the range, typically approximately 150 lbs. or more. However, the nature of a lead screw is that it has a constant mechanical advantage at all times. The lead screw must then be designed with a great mechanical advantage to accommodate the high forces at the end of the range. Consequently, the pull per turn is reduced. Several turns of the screw are therefore required. It is not uncommon for between 4 and 6 full turns of the screw to be required in order to transition the device from a malleable state to a rigid state. This process is tedious, tiring, and time-consuming.
Moreover, during the time required to transition the malleable shaft into a rigid state, the position of the stabilizer positioned on an end of the malleable shaft may drift on the surface of the heart. To overcome this, it is also not uncommon for the surgeon to position the retractor and hold it in place, and then instruct an assistant to turn the handle the several turns required to tension the shaft.